Electron Paramagnetic Resonance (EPR) provides a sensitive means of detecting and quantitating free radical species. Conventional continuous wave (CW) EPR has provided insight into the basic chemistry of free radical reactions and has most recently been increasingly used to probe the intricacies of biological free radical intermediates. Although EPR is vastly more sensitive than NMR, presently its use to study biological tissue has been limited by the frequency used, routinely greater l GHz where penetration of the radiation is minimal. Because radiation induces a cascade of electrons and ultimately results in formation of free radical species, the RBB has been developing both a pulsed Fourier Transform (FT) EPR system for spectroscopy and Imaging. Since excited electrons relax in microseconds and less, the use of nanosecond FT-EPR with heretofore unachievable rapid signal sampling and averaging is required for detection of transient, short-lived free radical signals. Likewise, the previous technologic limitations imposed by CW-EPR microwave frequencies may be overcome when EPR at radiofrequencies with pulsing techniques with efficient data sampling/averaging are employed. The goals of this project are to create a prototypical FT-EPR instrument for in vitro biologic studies and to establish the foundation of FT-EPR in vivo imaging. The feasibility of this experiment at radio frequencies and one and two dimension images of electrons has been demonstrated. An efficient sampler/averager has been designed and tested which demonstrates that the signal intensity of the free radical species can be enhanced to significant extent by acquiring the signal and averaging in short time intervals (<is). These initial observations suggest feasibility of this strategy of free radical detection at frequencies which permit studies of large biologic samples without significant compromise in signal levels. Now the electronics and computation are being optimized to establish the minimum detectable quantities of free radicals. Such information will lead to the resolution in imaging and also the levels of the free radical probe to be utilized in spectroscopic and imaging experiments. With this information, imaging of phantom objects and small animals will be attempted with practically useful free radical probes.